Electrostatic recording processes and materials for chargeless electrophotography

ABSTRACT

A METHOD OF RECORDING WITHOUT EXTERNAL ELECTROSTATIC CHARGING WHEREIN AN UNCHARGED RECORDING MATERIAL HAVING RECORDING LAYER CONTAINING PARTICLES OF AN N-TYPE PHOTOCONDUCTOR DISTRIBUTED IN A HYDROPHILIC POLYMERIC BINDING AGENT CONSTITUTING ABOUT 50-97% BY WEIGHT OF THE RECORDING LAYER IS IMAGEWISE EXPOSED TO RADIATION AND IS THEREAFTER ELECTROPHORETICALLY DEVELOPED WITH A DISPERSION OF ELECTROSTATICALLY ATTRACTABLE SUBSTANCES IN AN ELECTRICALLY INSULATING LIQUID MEDIUM.

United States Patent 3,687,658 ELECTROSTATIC RECORDING PROCESSES ANDMATERIALS FOR CHARGELESS ELECTROPHOTOGRAPHY Karel Verhille,Mortsel-Antwerpen, Andr Jan Conix,

Antwerpen, and Robert Joseph Noe, Mortsel-Antwerpen, Belgium, assignorsto Agfa-Gevaert N.V., Mortsel, Belgium No Drawing. Original applicationJune 27, 1966, Ser. No. 560,900. Divided and this application Oct. 27,1969, Ser. No. 869,867 Claims priority, application Great Britain, June25, 1965, 27,129/65, 27,130/65; Apr. 14, 1966, 16,459/ 66 Int. Cl. G03g13/22 US. Cl. 96-1 R 11 Claims ABSTRACT OF THE DISCLOSURE A method ofrecording without external electrostatic charging wherein an unchargedrecording material having recording layer containing particles of ann-type photoconductor distributed in a hydrophilic polymeric bindingagent constituting about 50-97% by weight of the recording layer isimagewise exposed to radiation and is thereafter electrophoreticallydeveloped with a dispersion of electrostatically attractable substancesin an electrically insulating liquid medium.

This application is a division of US. application Ser. No. 560,900,filed June 27, 1966, now abandoned.

The present invention relates to an electrostatic recording process. Itrelates especially to the production of images by means of exposed butnon-pre-charged photoconductive materials suited for that purpose.

The production of visible images by electrophotography is well known. Inthis system of photography, a photoconductive layer is provided with asurface electrostatic charge. On exposure to a radiation pattern, theelectric charges are dissipated in proportion to the intensity of theradiation.

The so formed charge pattern can be made visible by a specialdevelopment or toning technique based on the electrostatic attraction offinely divided particles. Well-established methods of developing theelectrostatic latent image include cascade, powder cloud, powdersuspension, magnetic brush, and fur-brush development. These are allbased on the presentation of charged toner to the surface bearing theelectrostatic image, Where coulomb forces attract the toner from carrierparticles or a liquid dispersing medium. The use of an electrophoreticdeveloper wherein an insulating liquid medium contains dispersedelectrostatically attractable particles is particularly interesting forthe development of electrostatic charge images which possess no veryhigh charge density and can be used, if the dispersed charged particleshave colloidal dimensions, for very-high-resolution work.

Besides the above described technique of forming an electrostatic latentimage, an electrostatic latent image may also be formed starting from aso-called latent electron image also known as a latent conductivityimage which is produced e.g. by normal exposure of a darkadaptedphotoconductive zinc oxide-insulating binder containing layer. Thelatent electron image is made up of excited electrons and bound holes.At the termination of the exposure an equilibrium exists between theelectrons in the conduction band and the trapped charge carriers. Byapplying an appropriate external field, those few electrons present inthe conduction band may be extracted from the layer. As they leave, theelectrons in the shallow traps are thermally excited into the conductionband from which,

Patented Aug. 29, 1972 "Ice in turn, they may also be extracted from thematerial to a contacting positive electrode.

So, in the latter system no precharging of the photoconductive materialbut the application of an electric field during or after the exposure isnecessary to form a detectable latent electrostatic image.

A method of recording and reproducing information has now been found,which method comprises record-wise or image-wise irradiating anon-precharged recording material, which in addition to an n-typephotoconductor incorporates a substance e.g. a polymeric substanceacting as a binder preferably a hydrophilic polymer, which substancecontains one or more groups selected from the class consisting of nitrogroups, nitroso groups, aldehyde groups, acyl groups, e.g. acetylgroups, carboxylic acid anhydride groups, carboxylic acid groups in acidor salt form, amido groups, e.g. carbonamido groups, ureido groups,hydroxyl groups and cyano groups, which groups accept electrons set freefrom said photoconductor, whereby a positive charge pattern incorrespondence with the exposed areas is formed by that irradiatingalone, and developing the exposed material by electrostaticallyattractable substances, preferably finely dispersed particles in aninsulating liquid or finely divided particles applied from an aerosol.

The content of such polymeric substance may vary from 50% to 3% byweight of the recording layer of the recording material.

By record-wise or image-wise exposing to electromagnetic radiation ismeant that the exposure may be progressive (in the sense that therecording of spoken information on a recording tape is progressive), orsimultaneous, e.g. as is the case of reflectographic or transmissionexposure respectively to or through an original, e.g., a printed text orsilver image transparency.

The recording process according to the present invention can besuccessfully applied for the reproduction in enlarged form of microfilmtransparencies and X-ray photography.

As n-type photoconductor e.g. titanium dioxide and zinc oxide butpreferably photoconductive zinc oxide is used.

Other n-type photoconductive substances are described Zeitschr. fiirPhysik (1936) 5079Tabelle 2, p. 61-62.

n-Type photoconductors such as zinc oxide and titanium dioxide areespecially sensitive to UV. and X-ray electromagnetic radiation andtheir sensitivity to visible light can be increased or extended bysensitization.

Polymeric substances which have proved to be particularly suited for usein combination with photoconductive zinc oxide and titanium dioxide inorder to obtain, by simple exposure and without precharging a positivecharge image of improved intensity are those containing one or more ofthe cited groups in at least a part of their structural units and whichhave a hydrophilic character. Polymers which are soluble in pure wateror in an alkaline aqueous liquid are preferably used. Some of them canbe cured by heating which can be done after coating.

The following is an illustrative list of such preferred polymers for useas binders:

Resins or condensates known as formaldehyde resins, e.g. urea, phenol,and melamine resins, containing methylol groups,

Resins containing carboxylic acid groups, in free, or salt form e.g. inammonium salt form, e.g. crotonic acid polymers more particularlycopoly(vinyl acetate/crotonic acid) and copoly styrene/crotonic acid,

Polymers containing hydroxyl groups e.g. vinyl alcohol polymers,methylol melamine resins and polysaccharides e.g. starch, alginic acid,cellulose and derivatives of said products wherein still hydroxy groupsare present,

Polymers containing amido and/or ureido groups, e.g. protein colloidssuch as gelatin.

As resins suitable for use according to the present invention may bemore particularly mentioned formaldehyde condensation products e.g.those listed in Table 1.

TABLE 1 (l) Parez Resin 613 (a dimethyltrimethylolmelamine marketed byAmerican Cyanamid Company).

(2) Parez Resin 607 (a melamine-formaldehyde resin marketed by AmericanCyanamid Company).

(3) Epok-W-980l (a 72 to 75% aqueous solution of a melamine-formaldehyderesin marketed by British Resin Products Ltd., London).

(4) Cymel 405 (a melamine-formaldehyde resin marketed by AmericanCyanamid Company).

(5) Resloom HP (a methylolmelamine marketed by Monsanto ChemicalCompany).

(6) Cassurit-MLP (partially etherified melamineformaldehyde resinmarketed by Cassella 'Farbwerke Mainkur A. G. Frankfurt(Main)--Fechenheim, Germany).

(7) Urecoll P (urea formaldehyde resin marketed by Badische Anilin &Sodafabrik Ludwigshafen/Rh.--Germany).

Two basic types of melamine resins are commercially available for useaccording to the present invention: unmodified and modified (etherified)melamine-formaldehyde polymers. Both types are marketed as spray-driedpowders and as solutions of about 60% of solid resin in water. Thesolutions may contain some alcohol for improvement of the storagestability. Spray-dried resin powders are produced under the trade nameCymel by American Cyanamid Company, New York, N.Y., U.S.A. and under thetrade name Resimene by Monsanto Chemical Company, St. Louis, Mo., U.S.A.Melamine resin solutions are marketed under the trade name Catalin Resinby Reichhold Chemicals Inc., White Plains, N.Y., U.S.A.

The water-soluble melamine-formaldehyde resins contain at least 2 molesof formaldehyde per mole of melamine.

As suitable melamine-formaldehyde resins containing several N-methylolgroups may further be mentioned: dimethylolmelamine (e.g. Resloom HP ofMonsanto Chemical Company); dimethyltrimethylolmelamine (Aerotex.

M3 or Parez 613 of American Cyanamid Company); trimethylolmelamine(Aerotex 605 or Parez 607 of American Cyanamid Company);tetramethylolmelamine (Resloom HP Special, Monsanto Chemical Company);tetramethylpentamethylolmelamine (Resloom LC-48, Monsanto ChemicalCompany); tetramethylhexamethylolmelarnine andpentamethylhexamethylolmelamine.

Further may be mentioned polymers containing carboxylic anhydride and/orcarboxylic acid groups in acid or salt form e.g.:

(1) copoly(vinyl acetate/crotonic acid) (94.4/ 5.6)

(2) copoly(vinyl acetate/methacrylic acid) (95/5) (3) alkyd resinscontaining carboxylic acid groups in acid or salt form (4) copolymers ofmaleic anhydride.

Polymers containing hydroxyl groups:

( 1) dextrine (2) potato starch (3) Solvitose H 4 (a hydroxyethyl starchmarketed by Scholten N.V. FoxholNetherlands; Molar substitution degreeof hydroxyethyl groups (MS) =0.27).

(4) Ceron--A (a hydroxyprqpylstarch marketed by Hercules Powder CompanyInc., Wilmington, Del. U.S.A., viscosity of a 5% aqueous solution at C.=500 c.p.).

(5) Sumstar J (a dialdehydylstarch marketed by Miles Interchemical-U.S.A.; the hydroxyl groups being for 94% substituted by aldehyde).

'(6) Carboxymethyl cellulose (7) CollateX-EH (amine-aliginate marketedby Alginate Ind. Ltd. London Eng.)

(8) Olgitex-768 (hydroxypropyl alginic acid marketed byHenkel--Germany).

(9) Arabic gum (10) Polyviol W 28/20 [copoly(vinyl alcohol/vinylacetate) (98/2) marketed by Wacker Chemie G.m.b.H. Miinchen, Germany].

The order of mixing steps for the constituents to form the coating isnot important. The kind of photoconductor does not play a part; anyknown photoconductor, inorganic or organic, may be used. Since knownphotoconductive substances are not water-soluble they have to bedispersed preferably with a dispersing agent that does not markedlylower the dark-resistivity of the photoconductive element.

The quantitative ratios of the photoconductive substances to the bindingagents may vary within wide limits. It is preferred to apply thephotoconductive substance in a ratio of 1 part by weight ofphotoconductive substance to 0.1 to 0.6 part by weight of total contentof binder.

The coating mixture may contain a dispersed n-type photoconductor in aweight ratio of 97% to 50% in respect of the total solids content of thecoated and dried layer.

The thickness of the photoconductive layer may be chosen between widelimits according to the requirements of each case. Good recording andreproduction results are attained with electrophotographic layers of athickness between 1 and 20 and preferably between 3 and 10 Thephotoconductive recording layers prepared according to the presentinvention may contain, in additon to the photoconductive substance(s)and binder optical sensitizers, e.g., those mentioned in Belgian patentspecification 612,102 :filed Dec. 29, 1961 by Gevaert Photo- ProductenN.V. and additives known in coating techniques, e.g., pigments (see e.g.Belgian patent specification 609,056 filed Oct. 12, 1961 by GevaertPhoto-Pro dncten N.V., compounds influencing the gloss and/or theviscosity, and compounds which counteract ageing and/ or oxidation ofthe layers, or which influence the thermal stability of the layers.

A very substantial increase in image density, which is probably due toan increase in sensitivity, can be obtained by the use in thephotoconductive recording element, preferably in combination with theabove mentioned resins and polymers of substances increasing thedarkresistivity as e.g. described and claimed in Belgian patentspecification 612,102 filed Dec. 29, 1961 by Gevaert Photo- ProductenN.V.

Said substances are preferably added in an amount of 10 mg. to 10 g. inrespect of 1000 g. of the photoconductor, although larger amounts may bepresent.

Whereas those compounds do effectively lower the electrical conductivityof the zinc oxide, it is clear that they also lower the concentration offree electrons in the recording element. 'It is believed that as aconsequence hereof a greater amount of electron accepting levels in thesystem composed of the binder and the photoconductor are not occupied;the absorption of light by the photoconductor and the inherent creationof hole-electron pairs thus leads to an increased electron capture andincreased sensitivity of the recording element.

Compounds which are especially useful to yield improved image recordingresults in combination with the electron-accepting binder are monomericcarboxylic acids preferably containing at least one hydroxyl group, and/or keto group and/ or carbonamido group. When the carboxylic acidcontains a hydroxyl group, said group preferably stands in theot-position with respect to the carboxylic acid group.

In the following Table 2 compounds are listed which yield improved imagerecording results (an increase in image density) when incorporated inthe photoconductive recording element.

TABLE 1 1. coon CHz-OH HOOc-( -ooon HC on C II H3C(|JC0OH OH 4 H2C-(|3H2o-c=o HOOC- c0orr COOH 8. OHz-CHg-CHg-CO o o. coon c0011 coon OHQOHQOH-0Hi CH2 CH2- L J 10. HO(CH2)10C OOH 11. crr-cn ll ii on COOH I coon N/1'1 13. (FE:

no CH-CHz-CHz-COOH The photoconductive zinc oxide need not necessarilybe prepared by the French process. Any type of photoconductive zincoxide may be used. So, it has been stated that direct process zinc oxidealso known as zinc oxide prepared according to the American process issuited and more particularly that photoconductive zinc oxide having ahigh water adsorption power (0.2 by weight) otfers very good results.Further it is not necessary to use a very pure zinc oxide since zincoxide e.g., doped with cadmium or gallium trioxide has proved to besuited as well.

The photoconductive layers for use according to the present inventionpreferably have a hydrophilic character as can be seen from the type ofsome of the binding agents mentioned above and the binder need notnecessarily be electrically insulating or more insulating than thephotoconductive zinc oxide. Owing to their hydrophilic character and theions which are incorporated in the coating composition e.g. fordispersing purposes said layers are as compared with the classicalelectrophotoconductive recording layers fairly electrically conductivealso in the dark. However, for obtaining a good contrast and highdensity the recording materials for use according to the invention arepreferably kept in the dark for a few days before image wise exposure.

Dispersing agents which can be used in the preparation ofphotoconductive recording layers containing zinc oxide or titaniumdioxide for use according to the present invention are e.g. sodiumhexametaphosphate, propionic acid, and monobutylphosphate.

The photoconductive layer may be coated dependent on the bindercomposition from an organophilic or an aqueous medium. When it is coatedfrom an aqueous medium the binding agent is preferably dissolved in anaqueous alkaline medium e.g. in aqueous ammonia, so that ammonium ionsare left to some extent in the recording layer after drying.

The photoconductive layer is preferably applied to a support in such anamount that between 8 and g. is present per sq. m. of dried coatingcomposition.

The electric properties of the support for the photo conductive layer donot play a fundamental role in the process of image recording accordingto the present invention. The recording process of the present inventioncan be performed with a recording layer coated on an electricallyinsulating support as well as with a recording layer coated on aconductive support. Thus, contrasty and dense images can be obtainede.g. by electrophoretic development of a charge pattern present in arecording layer applied to an insulating resin support (specificresistivity 10 ohm cm.) as well as with a charge pattern formed bysimple exposure and without precharging in a recording layer applied toa conductive metal or paper support.

Suitable electrophoretic developers for detecting the charge patternobtained by exposure alone without precharging the recording layer areof the negative type. Such developers are e.g. described in thepublished Belgian patent specification 650,423 filed July 10, 1964 byCommonwealth of Australia, the UK. patent specification 897,903 filedDec. 30, 1958 by Commonwealth of Australia, Belgian patent specification594,907 filed Sept. 6, 1960 by Laboratories of Australia, UK. patentspecifications 902,928 filed Sept. 18, 1958 by Commonwealth of Australiaand 1,016,072 filed Nov. 8, 1961 by General Aniline & Film Corporation.The electrophoretic development may be carried out in the presence of anelectric field e.g. as described in the Australian patent specification227,951 filed Apr. 5, 1957 by Commonwealth of Australia.

Another development technique which has proved to be suited fordeveloping rather weak charge images is known under the nameaerosol-developmen which includes the development with a smoke or mist.For such type of development reference is made e.g. to the US. patentspecifications 2,297,691 and 2,551,582 issued Oct. 6, 1942 and May 8,1951 both by Chester F. Carlson.

Although the working principles of the production of the positive chargepattern cannot be explained with complete certainty it is assumedwithout limiting the invention hereby that at the exposure the n-typephotoconductor e. g. photoconductive zinc oxide effects the followingphoto-reaction:

Before exposure the electron'accepting groups in the recording layerstand in equilibrium with the free electrons contained in the recordingelement. On exposure this equilibrium is disturbed since theconcentration in free electrons altered. Whereas the temperature andtrap density are not changed it is to be expected that a certainfraction of the quantity of electrons set free by the lightenergyabsorbed in the photoconductor has to be trapped in the various levelspresent in the recording material.

It is quite evident that upon capturing a portion of the createdphoto-electrons, a positive charge must remain on the surface of therecording element. Since in a n-type photoconductor the mobility of theelectrons usually is of a higher magnitude than the hole mobility, theelectrons diffuse further into the layer than the holes. Whereas theelectrons are captured in the recording element and are only slowlyexcited out of their capturing levels, a positive charge must beacquired by the surface of the recording element.

It is surprising however, that the trapping of these electrons, is arather rapidly occurring phenomenon whereas the inverse phenomenonrequires, as can be observed from the slow decay of the positive charge,a much longer period of time.

A method which has proved itself to be quite suited to detect this smallsurface charge to which the developing toner is attracted, consists inmeasuring the contact potential difference of the layer before and afterirradiation with respect to some standard material e.g. stainless steel(18/ 8).

The following examples illustrate the present invention without howeverlimiting it thereto.

EXAMPLE 1 g. of Resloom HP (trade name for a di-methylol melamine resinmarketed by Monsanto Chemical Company, Springfield, Mass, U.S.A.) weredissolved in 2.4 liter of water whereupon 600 ccs. of ethanol and 12ccs. of a 10% aqueous solution of sodium hexametaphosphate were added.Then 750 g. of Blane de Zinc, Neige extra pure, Type A (trade name forzinc oxide prepared by the French process marketed by Vieille MontagueS.A., Liege, Belgium) were added to the solution obtained. The mixturewas ground in a ball-mill for 24 hours.

The dispersion obtained was coated on a subbed cellulose triacetatesupport pro rate of 10 g. of zinc oxide per sq. m. The material formedwas exposed for 10 seconds through a negative, which was in contact withsaid material, by means of incandescent bulbs (together 450 watt) placedat a distance of 25 cm.

Immediately after exposure the material was developed for 40 sec. in anegatively charged dispersion of carbon black in a hydrocarbon mixture.Such a developer is the developer No. 3 described in the Belgian patentspecification 650,423 filed July 10, 1964 by Commonwealth of Australia.A very contrasty image was obtained.

When development takes place 15 minutes after the exposure, still a goodimage is obtained. When keeping the exposed material 1 hour in the darkbefore developing, the contrast is markedly decreased and when thestorage in the dark between development and exposure is 1 day no imageis obtained anymore.

EXAMPLE 2 Example 1 was repeated with the difference, however, that forforming the binding agent 150 g. of Resloom HP (trade name) and 150 ccs.of a 5% aqueous solution of Polyviol W 28/20 (trade name for apoly/(vinyl alco- 1101) containing 97.5 to 99.5 of vinyl alcohol groupsmarketed by Wacker-Chemie G.m.b.H., Miinchen, Germany) are used. Theresults obtained were as good as those of Example 1.

EXAMPLE 3 Example 1 was repeated with the difierence, however, that thecellulose triacetate support was replaced by glazed paper of 90 g./sq.m., coated with a gelatin interlayer.

The results obtained were as good as those of Example 1.

EXAMPLE 4 Example 1 was repeated with the difference, however, that thecellulose triacetate support was replaced by a glassine-type papersupport of 65 g./sq. m.

The results obtained were as good as those of Example 1.

EXAMPLE 5 Example 1 was repeated with the difference, however, that tothe coating composition 0.5 cc. of a solution in water of cerium(1V)sulphate in respect of 25 g. zinc oxide was added. The obtained imageshowed an improved contrast.

EXAMPLE 6 Example 1 was repeated with the difierence, however, that tothe coating composition 4.5 ccs. of a 5% solution in ethanol offuranwa-carboxylic acid calculated on 22.5 g. of zinc oxide was added.For obtaining a same image density as obtained with the material ofExample 1 the exposure time could be reduced to 2.5 seconds.

EXAMPLE 7 Example 1 was repeated with the difference, however, that tothe coating composition 0.4 cc. of a 5% solution in ethanol ofot-N-Cil'l'lC acid monododecylamide was added. For obtaining a sameimage density as obtained with the material of Example 1 the exposuretime could be reduced to 2.5 seconds.

EXAMPLE 8 Example 1 was repeated with the difference, however, that thephotoconductive layer was composed of the following mixture:

hydroxyethylstarch (MS of hydroxyethyl groups: 0.27):

photoconductive zinc oxide: 10 g.

a mixture of water and ethanol (80/20): 50 ccs.

which was ground in a ball-mill before coating. The exposure was carriedout as described in Example 1 but the exposure time was doubled. Onelectrophoretic development a contrasty image was obtained.

EXAMPLE 9 5 g. of a novolac (phenolformaldehyde resin with melting point160-170' F.) was diluted with 20 ccs. of cyclohexanon and 80 ccs. oftoluol and mixed in a ball-mill with 30 g. of photoconductivezinc-oxide.

The ground composition wascoated on a baryta-coated paper pro rata of 20g. per sq. m. of solid substance. The processing is the same asdescribed in Example 1 with the difference, however, that an exposuretime of 1 min. was applied.

EXAMPLE 10 150 g. of Vinac ASB (trade name for a copoly(vinylacetate/crotonic acid) (94.4/5.6) marketed by Colton Chem. Co. adivision of Air Reduction Co., Inc., Cleveland, Ohio, U.S.A.) weredissolved in a mixture of 3.2 litres of water, 800 ccs. of ethanol and 4g. of concentrated aqueous ammonium hydroxide. Then 3 g. of sodiumhexamethaphosphate as a dispersing agent dissolved in 30 ccs. of water,were added whereupon 750 g. of photoconductive zinc oxide preparedaccording to the American Process were admixed while strongly stirring.The mixture was ground for 20 hours in a ball-mill.

10 The mixture was then coated on a cellulose triacetate support asdescribed in Example 1. Exposure and development of the materialobtained occurred as described in Example 1 with the diiference,however, that the exposure time was 20 seconds.

EXAMPLE 11 A mixture was made of the following ingredients:

75 g. of Resloom HP (trade name) in 500 ccs. of water g. of Urecoll P(trade name for a urea-formaldehyde resin marketed by Badische Anilin &Soda-Fabrik Ludwigshafen/Rh.Germany) in 600 ccs. of Water,

1100 ccs. of water 800 ccs. of ethanol 24 ccs. of a 10% aqueous sodiumhexametaphosphate solution.

While strongly stirring, 1200 g. of photoconductive zinc oxide wereadded and the whole was ground for 18 hours in a ball-mill. Then cos. ofwater were added.

The dispersion obtained was coated by dip-coating pro rata of 19 g. ofdry substance per sq. m. on a glazed paper support of 90 g. per sq. 111.provided with a gelatin interla er.

A silver image transparency was placed in contact with thephotoconductive layer and direct exposure for a period of 0.005 sec.carried out with an 80 watt high pressure ultra-violet radiation bulb(marketed under the name U.V. lamp HP 80 by Philips Gloeilampen fabriekN.V., Eindhoven, Netherlands) placed at a distance of 25 cm. A verycontrasty image was obtained with the developer number 3 described inthe published Belgian patent specification 650,423 filed July 10, 1964by Commonwealth of Australia.

EXAMPLE 12 300 g. of photoconductive zinc oxide prepared by the Frenchprocess were ground in a ball-mill for 16 hours in the presence of 1litre of a 1% aqueous solution of gelatin. The ground composition iscoated onto a glassine type paper support of 60 g. per sq. m. at such arate that 15 g. of solid substances were present after drying.

The image-wise exposure is carried out through a negative transparencywith an ultra-violet lamp of 80 watt placed at distance of 15 cm. andirradiating the photo sensitive n aterial for 15 seconds. Development iscarried out with the developer described in Example 1.

By adding to the above composition 60 ccs. of a 5% solution in ethanolof one of the compounds mentioned with their structural formula in theTable 4 a higher density was obtained.

EXAMPLE 13 300 g. of titanium dioxide (TiO were ground in a ballmill for15 h. in the presence of 1 litre of a 1% aqueous solution of gelatin.The ground composition is coated onto a glassine type paper support of60 g. per sq. m. at such a rate that 15 g. of solid substances werepresent after drymg.

The image-wise exposure is carried out through a negative transparencywith an ultra-violet lamp of 80 watt placed at a distance of 15 cm. andirradiating the photosensitive material for 5 seconds. Development iscarried out with the developer described in Example 1.

A cllegible sufiiciently dense and contrasty image is obtaine EXAMPLE 14g. of Resloom HP (trade name for a di-methylol melamine resin marketedby Monsanto Chemical Company, Springfield, Mass., USA.) were dissolvedin 2.4 litres of water whereupon 600 ccs. of ethanol and 12 cos. of a10% aqueous solution of sodium hexametaphosphate were added. Then 750 g.of Blane de Zinc, Neige extra pure, Type A (trade name for zinc oxideprepared by the French process marketed by Vieille Montague S.A., Lige,

Belgium) were added to the solution obtained. The mixture was ground ina ball-mill for 24 hours.

The dispersion obtained was coated on a subbed cellulose triacetatesupport pro rata of 10 g. of zinc oxide per The material obtained wasdirectly X-ray irradiated for 1 min. through an aluminium wedge, or theexposure carried out for the same period by means of an intensifyingscreen (fluorescence maximum at 500 mg). The exposure characteristics ofthe X-ray radiation bulb are 120 RV. and 5 A. A visible printcorresponding to the Wedgedensities can be obtained by a liquiddevelopment using an insulating liquid as described in the Australianpatent specification 227,951 filed Apr. 5, 1957 by Commonwealth ofAustralia.

The electrophoretic developing composition for use according to thepresent invention contains electrostatically attractable substancesdispersed in an insulating (preferably at least ohm cm.) liquid medium.For other developing compositions reference is made to the UK. patentspecification 897,903 filed Dec. 30, 1958 by Commonwealth of Australia,Belgian patent specification 594,- 907 filed Sept. 6, 1960 byLaboratories of Australia and UK. patent specification 902,928 filedSept. 18, 1958 'by Commonwealth of Australia.

We claim:

1. A method of recording and reproducing information, which methodcomprises the steps of (1) image-wise irradiating an uncharged recordingmaterial having a recording layer containing an admixture of an n-typephotoconductor with a hydrophilic polymer which accepts electrons setfree from said photoconductor on irradiation to active electromagneticradiation and is selected from the class consisting of polymerscontaining hydroxyl groups, ureido groups, amido groups, tarboxylic acidgroups in acid or salt form, acyl groups and aldehyde groups, the amountof said photoconductor being about 50-97% by weight of the total drysolids content of said recording layer, to produce in such layer apositive charge pattern corresponding to the irradiated areas thereofand (2) electrophoretically developing the positive charge pattern incorrespondence with the exposed areas by contacting said layers with anelectrophoretic developer consisting essentially of negatively chargedtoner particles dispersed is an electrically insulating liquid medium.

2. A method according to claim 1, wherein the record ing material isproduced by coating onto a support a dispersion of said photoconductorin an aqueous medium containing the polymer in dissolved state.

3. A method according to claim 2, wherein the aqueous medium containsammonia.

4. A method according to claim 1, wherein said polymer is substantiallywater-soluble.

5. A method according to claim 4, wherein said polymer is amelamine-formaldehyde resin, urea-formaldehyde condensate orphenolformaldehyde condensate capable to be dissolved in water or anaqueous alkaline medium.

6. A method according to claim 1, wherein the n-type photoconductor isphotoconductive zinc oxide.

7. A method according to claim 1 wherein the recording material inadmixture with the n-type photoconductor contains positive metal ionshaving strong electron capturing character in their highest oxidationstate.

8. A method according to claim 7, wherein the said ions are cerium(IV)ions.

9. A method according to claim 1, wherein said polymer is polyvinylalcohol.

10. A method according to claim 1, wherein said polymer containssuflicient methylol groups as to be water soluble.

11. A method according to claim 1, wherein the recording materialcontains in addition to the n-type photoconductor a monomeric carboxylicacid compound containing a group selected from the class consisting ofhydroxyl, keto and carbonamido.

References Cited UNITED STATES PATENTS 2,956,874 10/1960 Giaimo 96-12,959,481 11/1960 Kacera 96-1 2,990,280 6/1961 Giaimo 96-1 2,997,.3 878/1961 Tanenbaum 96-1 3,121,006 2/ 1964 Middleton et a1. 96-1 3,159,48312/ 1964 Behanenberg 96-1 3,160,503 12/ 1964 Cady 96-1 3,408,183 10/1968'Mammino 96-15 3,447,957 6/1969 Behringer 117-201 JOHN C. COOPER III,Primary Examiner US. Cl. X.R.

